1. Field of the Invention
The present application relates to an over-current protection device, and more particularly to a surface-mountable over-current protection device.
2. Description of Related Art Including Information Disclosed Under 37 CFR 10.97 and 37 CFR 1.98
Over-current protection devices are used for protecting circuitries from damage resulted from over-heat or over-current. An over-current protection device usually contains two electrodes and a resistive material disposed therebetween. The resistive material has positive temperature coefficient (PTC) characteristic that the resistance thereof remains extremely low at room temperature and instantaneously increases to thousand times when the temperature reaches a critical temperature or the circuit has over-current, so as to suppress over-current and protect the cell or the circuit device. When the resistive material gets back to the room temperature or over-current no longer exists, the over-current protection device returns to be of low resistance and as a consequence the circuitry again operate normally. In view of the reusable property, the PTC over-current protection devices can replace traditional fuses, and have been widely applied to high density circuits.
Referring to FIG. 1A, U.S. Pat. No. 6,377,467 disclosed a surface-mountable over-current protection device 10 containing a resistive device 11, a first electrode 17, a second electrode 18, insulating layers 15 and 16, a first conductive via 19 and a second conductive via 20. The resistive device 11 contains a first conductive member 13, a second conductive member 14 and a PTC material layer 12. The PTC material layer 12 is stacked between the first conductive layer 13 and the second conductive layer 14, and extends along with the conductive members 13 and 14 in the horizontal direction to form a laminated structure. The conductive vias 19 and 20 extend vertically, and may be plated through holes (PTH) formed by laser or mechanical drilling to connect the conductive layers 13, 14 and the electrodes 17, 18. Owing to small contact areas of the PTH 19 and 20 and the conductive layers 13 and 14, the contact resistances are large. Therefore, it is difficult to further decrease the resistance of the device.
To avoid circuit short between conductive via 19 and the conductive member 14 or the conductive via 20 and the conductive member 13 caused by the event of drilling misalignment, the conductive members 13 and 14 have to be apart from the corresponding lateral surfaces. FIG. 1B exemplifies that conductive member 13 is apart from the conductive via 20. Alternatively, the conductive members 13 may have an edge surrounding the via 20 to avoid circuit short, as shown in FIG. 1C. As a consequence, the effective area of the resistive device 11 is, however, diminished, inducing high resistance of the over-current protection device 10.